18/6/2020
This article (Haan and Dijkerman 2020) is an updated version of a previous review (Dijkerman and Haan 2007) (cited 322 times according to Scopus) where Dijkerman and De Haan proposed a somatosensory model inspired by the two-route model of visual system (Goodale and Milner 1992).
Based on:
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An ascending system, the dorsal column–medial lemniscus pathway, from the somatosensory receptors (Paccini, Merkel, Ruffini, Meissner) to the dorsal column in the spinal cord and subsequently arrives in the medulla where they decussate and terminate in the Thalamus (ventral posterior lateral nucleus). Then, from the Thalamus to SI. |
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| A second ascending system, the anterolateral system, mainly deals with thermal and noxious stimuli and some tactile inputs. It also projects to the Thalamus. |
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There is convincing evidence of crosstalk between pathways.
Dissociations between the two routes depend on the precise task conditions and there are now many examples of ‘perceptual information’ affecting action programming and vice versa. ( RHI impact on movement displacements and grasping actions )
Somatosensory processing is not only for perception and the guidance of action.
Somatosensory processing ontributes to functions that are:
examples:
Reviewing the Literature, the Authors observed that:
1- there are observation concerned structures that they had omitted in the first model 2- these structures are all highly interconnected
This suggests either one comprehensive or several overlapping networks.
Therefore, the Authors identified five main networks in a interactive, distributed network.
Cylinder block:
basic somatosensory processing unit dominated by somatosensory input.
in SI simple features, such as stimulus location and duration, that lead to direction and velocity of a target moving on the body surface.
This Cylinder block provides the input for 5 networks:
Recognizing objects needs:
the insular cortex is NOT involved in object recognition or that its role is limited to the affective aspects of objects
in addition to posterior parietal areas, the frontal lobes also play a role in haptic recognition beyond programming exploratory movements
prefrontal and parietal involvement in long-term tactile memory as well as a multimodal memory representation.
vision + somatosensory: activations in the medial and superior frontal gyrus and the superior parietal lobe including the intraparietal sulcus.
Imaging tactile features of an object accelerate reaction times to tactile but not auditory stimuli
Tactile imagery activation in: - SI - retrosplenial cortices - precuneus - prefrontal cortex \(\leftarrow\) top-down activation of a visual representation of a haptically explored object: haptic object recognition is multimodal!
Body perception depends on basic and higher-order representations.
Structural and spatial representations are dissociable but they share underlying neural networks and show related functional representations.
behavioural study suggests that there is a mental body representation structured in categorical body parts, with the joints as boundaries
structural aspects of body representation affect early somatosensory processing (N80 from controlateral SI)
finger agnosia and left-right disorientation: inferior parietal lobe damage, right anterior insula
Spatial aspects of body representations often studied with tactile distance estimation task (Spitoni et al. 2013).
Using the tactile distance estimation task it was observed the activation of:
Thus, parietal areas appear to be mainly responsible for processing spatial input.
Definition: the feeling that a body (or part of it) belongs exclusively to you
three networks contribute in anosognosia for hemiplegia: the premotor loop, the limbic system and the ventral attentional network (Pacella et al. 2019)
somatoparaphrenia mainly occurs after right hemispheric damage, usually posterior (temporoparietal) or insular but occasionally subcortically (Vallar and Ronchi 2009)
in somatoparaphrenia are involved frontal (middle and inferior frontal gyrus) and subcortical (amygdala, hippocampus, thalamus, basal ganglia, white matter, internal capsule posterior limb) structures in addition to the postcentral gyrus (Moro et al. 2016)
RHI: ventral premotor and posterior parietal areas, the EBA, the cerebellum, and the putamen to be involved + insula (Ehrsson, Spence, and Passingham 2004)
An important, but not the only, sensory channel for the positive affective aspect of touch is the c-tactile system
Unmyelinated c-fibres in hairy skin respond best to slow (1–10 cm/s) stroking with a soft brush or hand and at skin temperature (about 34°C), suggesting sensitivity for skin-on-skin contact (Rolls 2010).
These fibers directly aim to the Insular cortex.
The areas activated by affective touch are dissociable, but also overlapping the areas activated by disctiminatory tactile stimuli.
Disciminative touch: SI, SII, the left lateral inferior premotor cortices, the inferior parietal cortex, the SMA, and bilateral angular gyri and medial prefrontal areas.
Affective touch: bilateral network posterior and anterior insula, the postcentral primary and secondary somato-sensory cortex, the putamen, the thalamus, the frontal operculum, and the medial prefrontal cortex.
This suggests that the anterior insula is particularly important for the affective interpretation of touch.
There are many other tactile stimuli that are socially relevant:
Somatosensory (proprioception) information is used for:
This model is an interactive, distributed framework.
It shows functional specificity, but somatosensory processing in its broadest sense involves a network of cortical and subcortical areas that shows a more flexible organization than the first model previously assumed.
Questions
A central thesis here is the multimodal aspect of the networks that are informed by somatosensory information. The mechanisms are not well understood.
Affective somatosensory processing is not reliant on C-fibres only, and we need to look into the inter-action between these systems.
Time course of the activation of the different networks.
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